Mine changes every few months, depending on energy levels. Right now it’s scrambled or boiled eggs with toast.
In the winter I was obsessed with making a cabbage & random vegetable soup. I just shredded the cabbage, cut up and sauteed various veggies in olive oil (carrots, potatoes, parsnips, celery root, etc), put in spices (pepper, thyme, paprika, bay leaf, and whatever else seemed fun), then added the cabbage and waited until it lost some volume, added hot water/stock, boiled for 30-40min. Towards the end I might add some bell peppers or broccoli or other vegetable with a short cooking time. After removing from the stove, I did a couple pulses with the immersion blender to thicken the soup while leaving whole veggie bit inside. Finally, I topped with lemon juice after serving.
One of my favorite snacks to eat on daytrips are hardboiled eggs. You could bring chili/spice mix packets with you to add flavor on the road (I like buying some labeled as “hot pot mate” from my local chinese market). Not much of a meal by themselves of course, but can be a very filling supplement to something like a sandwich or a salad. Good as in between snacks too.